Selected structurally defined novolak binder resins and their use in radiation-sensitive compositions

ABSTRACT

A novolak resin composition comprising at least one unit of the reaction product of a para-, para-bonded bisphenol having formula (A): ##STR1## wherein R 1  =hydrogen, lower alkyl group having 1-4 carbon atoms, halogen, or lower alkoxy group having 1-4 carbon atoms; 
     wherein R 2  =hydrogen or lower alkyl group having 1-4 carbon atoms; and 
     wherein X is selected from the group consisting of: CH 2 , CH(CH 3 ), C(CH 3 ) 2 , O, and S; 
     with a bismethylol monomer selected from a difunctional ortho-, ortho-phenolic bismethylol of Formula (B), a difunctional ortho-, para-phenolic bismethylol of Formula (C): ##STR2## wherein R 3  is selected from CH 3 , CH 2  CH 3 , Cl, and Br; and 
     wherein R 4  is selected from H and CH 3 .

This application is a division of application Ser. No. 7/787,454 filedNov. 4, 1991, now U.S. Pat. No. 5,306,594 which is incorporated hereinby reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to selected structurally defined novolakresins containing at least one unit which is a condensation reactionproduct of a selected para-, para-bonded bisphenol containing at leastone unsubstituted ortho position to the hydroxyl on each aromatic ringwith either a selected difunctional ortho-, ortho-phenolic bismethylol,or a selected difunctional ortho-, para-phenolic bismethylol, or acombination of such bismethylols.

Furthermore, the present invention relates to radiation-sensitivecompositions useful as positive-working photoresist compositions,particularly, those containing these phenolic resins ando-quinonediazide photosensitizers. Still further, the present inventionalso relates to substrates coated with these radiation-sensitivecompositions as well as the process of coating, imaging, and developingthese radiation-sensitive mixtures on these substrates.

2. Brief Discussion of the Prior Art

Photoresist compositions are used in microlithographic processes formaking miniaturized electronic components such as in the fabrication ofintegrated circuits and printed wiring board circuitry. Generally, inthese processes, a thin coating or film of a photoresist composition isfirst applied to a substrate material, such as silicon wafers used formaking integrated circuits or aluminum or copper plates of printedwiring boards. The coated substrate is then baked to evaporate anysolvent in the photoresist composition and to fix the coating onto thesubstrate. The baked coated surface of the substrate is next subjectedto an image-wise exposure of radiation. This radiation exposure causes achemical transformation in the exposed areas of the coated surface.Visible light, ultraviolet (UV) light, electron beam and X-ray radiantenergy are radiation types commonly used today in microlithographicprocesses. After this image-wise exposure, the coated substrate istreated with a developer solution to dissolve and remove either theradiation-exposed or the unexposed areas of the coated surface of thesubstrate. In some instances, it may be desirable to bake the imagedcoated substrate after the imaging step and before the developing step.This bake step is commonly called a post-exposure bake and is used toincrease resolution.

There are two types of photoresist compositions--negative-working andpositive-working. When negative-working photoresist compositions areexposed image-wise to radiation, the areas of the resist compositionexposed to the radiation become less soluble to a developer solution(e.g., a cross-linking reaction occurs) while the unexposed areas of thephotoresist coating remain relatively soluble to a developing solution.Thus, treatment of an exposed negative-working resist with a developersolution causes removal of the nonexposed areas of the resist coatingand the creation of a negative image in the photoresist coating, andthereby uncovering a desired portion of the underlying substrate surfaceon which the photoresist composition was deposited. On the other hand,when positive-working photoresist compositions are exposed image-wise toradiation, those areas of the resist composition exposed to theradiation become more soluble to the developer solution (e.g., arearrangement reaction occurs) while those areas not exposed remainrelatively insoluble to the developer solution. Thus, treatment of anexposed positive-working resist with the developer solution causesremoval of the exposed areas of the resist coating and the creation of apositive image in the photoresist coating. Again, a desired portion ofthe underlying substrate surface is uncovered.

After this development operation, the now partially unprotectedsubstrate may be treated with a substrate-etchant solution or plasmagases and the like. This etchant solution or plasma gases etch theportion of the substrate where the photoresist coating was removedduring development. The areas of the substrate where the photoresistcoating still remains are protected and, thus, an etched pattern iscreated in the substrate material which corresponds to the photomaskused for the image-wise exposure of the radiation. Later, the remainingareas of the photoresist coating may be removed during a strippingoperation, leaving a clean etched substrate surface. In some instances,it is desirable to heat treat the remaining resist layer after thedevelopment step and before the etching step to increase its adhesion tothe underlying substrate and its resistance to etching solutions.

Positive-working photoresist compositions are currently favored overnegative-working resists because the former generally have betterresolution capabilities and pattern transfer characteristics.

Photoresist resolution is defined as the smallest feature which theresist composition can transfer from the photomask to the substrate witha high degree of image edge acuity after exposure and development. Inmany manufacturing applications today, resist resolution on the order ofone micron or less is necessary.

In addition, it is generally desirable that the developed photoresistwall profiles be near vertical relative to the substrate. Suchdemarcations between developed and undeveloped areas of the resistcoating translate into accurate pattern transfer of the mask image ontothe substrate.

Increased resolution has been noted in positive photoresist systemswhose novolaks possess a high degree of ortho-, ortho-bonding. The termortho-, ortho-bonding is used to refer to the location and positions ofattachment of the methylene bridge between phenolic nuclei. Thus, thebridge which connects two phenolic nuclei which is ortho to bothphenolic hydroxyl groups is regarded as ortho, ortho.

It is thought that ortho-, ortho-bonding increases the interactionsbetween the novolak and the photoactive compound in positivephotoresists compared to positive photoresists containing novolaks whichlack a high degree of ortho-, ortho-bonding in their microstructure.Although the exact character of these interactions is speculative, e.g.,hydrogen bonding, van der Waals forces, and the like, there is acorrelation between increased resolution and contrast observed in thesepositive resists whose novolaks contain a high degree of ortho-,ortho-bonding compared to positive resists whose novolaks lack this highdegree of ortho-, ortho-bonding.

Furthermore, it is also known that the incorporation of trimeric ortho-,ortho-blocks into a novolak is a more efficient use of ortho-,ortho-bonding. See Honda et al, "Studies of Dissolution InhibitionMechanism of DNQ-Novolak Resist [II] Effect of Extended Ortho-Ortho Bondin Novolak", SPIE Vol. 1466, Advances in Resist Technology andProcessing VIII (1991), page 141 et seq. It is also believed thatdimeric ortho-, ortho blocks in the novolak aid in improving resistperformance. Monomeric ortho-, ortho-units and those larger thantrimeric are less effectual in improving resist performance. Thus, lessoverall ortho-, ortho-bonding content is needed when it is incorporatedin dimeric and trimeric form.

The present invention makes use of these dimeric and trimeric ortho-,ortho-blocks to achieve a novolak resin which can result in aphotoresist having advanced lithographic properties.

BRIEF SUMMARY OF THE INVENTION

One object of the present invention is directed to a new type of novolakresin with well-defined structural features which incorporates ortho-,ortho-bonding into the novolak resin in the form of dimeric or trimericunits or both types of units.

Another object of the present invention is to make a novolak resinwherein the above-noted dimeric or trimeric units are homogeneouslydistributed throughout the polymer.

Still another object is to make a novolak resin whereby the structure,ortho-, ortho-content, and block type are defined by a reactivecombination of monomeric precursors encompassing a mixture of:

(1) a para-, para-bonded bisphenol containing at least one unsubstitutedposition ortho to the hydroxyl on each aromatic ring; with

(2) either a difunctional ortho-, ortho-phenolic bismethylol, ordifunctional ortho-, para-phenolic bismethylol, or a combination of suchbismethylols.

And further, the present invention is directed to incorporating thosenovolak resins into photoresist formulations which will have improvedresolution due to the efficient incorporation of ortho-, ortho-bondingas well as having increased thermal resistance.

Accordingly, the present invention is directed to a novolak resincomposition comprising at least one unit of the reaction product of apara-, para-bonded bisphenol having formula (A): ##STR3## wherein R₁=hydrogen, lower alkyl group having 1-4 carbon atoms, halogen, or loweralkoxy group having 1-4 carbon atoms;

wherein R₂ =hydrogen or lower alkyl group having 1-4 carbon atoms; and

wherein X is selected from the group consisting of CH₂, CH(CH₃),C(CH₃)₂, O, and S;

with a bismethylol monomer selected from a difunctional ortho-,ortho-phenolic bismethylol of formula (B), a difunctional ortho-,para-phenolic bismethylol of formula (C), or mixtures thereof: ##STR4##wherein R₃ is selected from CH₃, CH₂ CH₃, Cl, and Br; and

wherein R₄ is selected from H and CH₃.

Moreover, the present invention is directed to a radiation-sensitivecomposition useful as a positive photoresist comprising an admixture ofo-quinonediazide compound and binder resin comprising at least one unitof the condensation product described above; the amount of saido-quinonediazide compound or compounds being about 5% to about 40% byweight and the amount of said binder resin being about 60% to 95% byweight, based on the total solid content of said radiation-sensitivecomposition.

Also further, the present invention encompasses said coated substrates(both before and after imaging) as novel articles of manufacture.

Still further, the present invention also encompasses the process ofcoating substrates with these radiation-sensitive compositions and thenimaging and developing these coated substrates.

DETAILED DESCRIPTION

The bisphenol monomers of formula (A) are selected para-, para-bondedbisphenols containing at least one unsubstituted ortho position on eacharomatic ring. The preferred bisphenol monomers are those wherein R₁=CH₃, H, Cl, Br, or OCH₃ ; R₂ =H or CH₃ ; and X=CH₂, CH(CH₃) or C(CH₃)₂.The most preferred bisphenol monomer is2,2"-bis(4-hydroxy-3-methylphenyl) propane (BHMPP) wherein R₁ =CH₃ ; R₂=H; and X=C(CH₃)₂.

These bisphenol monomers are either commercially available or may beprepared by known methods including the reaction of excess phenolic withan appropriate aldehyde or ketone.

The difunctional ortho-, ortho-phenolic bismethylol of formula (B) maybe any 2,6-bismethylol of a phenol with at least a nonreactive parasubstituent as defined by formula (B). The preferred difunctionalortho-, ortho-phenolic bismethylols have R₃ =CH₃ or CH₂ CH₃ and R₄ =H.The most preferred difunctional ortho-, ortho-phenolic bismethylol isp-cresol bismethylol (BHMPC) (wherein R₃ =CH₃ and R₄ =H). Otherillustrative difunctional ortho-, ortho-phenolic bismethylols includethe following:

The 2,6-bismethylol of 3,4-dimethylphenol, 4-ethylphenol,4-chlorophenol, 4-bromophenol, 3-methyl-4-chlorophenol,3-methyl-4-bromophenol, 3-methyl-4-ethylphenol, 3-chloro-4-ethylphenol,and 3-bromo-4-ethylphenol.

These difunctional ortho-, ortho-monomers are either commerciallyavailable or may be prepared by conventional processes for makingphenolic bismethylols.

The difunctional para-, ortho-phenolic bismethylols of formula (C) maybe any 2,4-bismethylol of a phenol with at least a nonreactive orthosubstituent as defined by formula (C). The preferred difunctional para-,ortho-phenolic bismethylol have R₃ =CH₃ or CH₂ CH₃ and R₄ =H. The mostpreferred difunctional para-, ortho-phenolic bismethylol is o-cresolbismethylol (BHMOC) (wherein R₃ =CH₃ and R₄ =H). Other illustrativedifunctional ortho-, para-phenolic bismethylols include the following:

The 2,4-bismethylol of 2,3-dimethylphenol, 2,5-dimethylphenol,2-ethylphenol, 2-chlorophenol, 2-bromophenol, 3-methyl-2-chlorophenol,3-methyl-2-bromophenol, 3-methyl-2-ethylphenol, 3-chloro-2-ethylphenol,3-bromo-2-ethylphenol 5-methyl-2-chlorophenol, 5-methyl-2-bromophenol,5-methyl-2-ethylphenol, 5-chloro-2-ethylphenol, and5-bromo-2-ethylphenol.

These difunctional para-, ortho-phenolic bismethylols are eithercommercially available or may be prepared (as described by F. S.Granger, Industrial and Engineering Chemistry, Vol, 24, No. 4, pp.442-448) by reacting the desired phenolic at room temperature with twomole equivalents of formaldehyde and one mole equivalent of sodiumhydroxide in the form of a 20% aqueous solution. After the reaction iscomplete, the solution is neutralized with acetic acid. Over time, thebismethylol crystallizes and is isolated by filtering, washing withwater, and drying in vacuo to constant weight.

The reaction of the monomers of formula (A) with the monomers of formula(B) form novolak resins which have only dimeric ortho-, ortho-blocks.For example, the reaction of BHMPP and BHMPC form the followingstructure (D): ##STR5##

The reaction of the monomers of formula (A) with the monomers of formula(C) form novolak resins which have only trimeric ortho-, ortho-blocks.For example, the reaction of BHMPP and BHMOC form the followingstructure (E): ##STR6##

The structure is so well defined that the weight percent dimer andtrimer ortho-, ortho-blocks can be easily calculated from the monomerfeed ratios. For example, polymerizing BHMPP with both BHMOC and BHMPCgives a novolak which contains both ortho-, ortho-dimer, and trimerblocks in known amounts. The content of the type of methylene bridging(ortho-ortho, ortho-para, or para-para) may also be exactly defined.Such calculations would be valid since all three monomer reactants areall difunctional and reaction proceeds unambiguously.

Preferred mole ratios of the monomers of formula (A) to the combinedmonomers of formulae (B) and (C) would be from about 1:0.8 to about1:1.2.

Preferred mole ratios of the monomers of formula (B) to formula (C)depends on the particular end use and the novolak alkali solubilityrequired for that use.

In making the present class of resins, the precursors, compounds offormulae (A), (B), and (C) are placed in a reaction vessel which alsocontains an acid catalyst and solvent. The mixture is then heated to atemperature in the range from about 60° C. to about 120° C., morepreferably from about 80°-110° C., for the condensation polymerizationprocess to occur. The reaction time will depend on the specificreactants and catalyst used. Reaction times from 1-24 hours aregenerally suitable. The reaction volatiles and solvent are then removedby distillation to yield the desired product.

Typical catalysts include oxalic acid, maleic acid, hydrochloric acid,sulfonic acids, and other acid catalysts known to those skilled in theart of novolak synthesis. Preferred catalysts include oxalic and maleicacid. The most preferred catalyst is oxalic acid. The acid catalystconcentration may range from about 0.1% to about 2%.

Solvents which may be employed are those of medium polarity which arenot extremely acid and/or water sensitive. Suitable solvents includeethereal solvents such as tetrahydrofuran (THF) and dioxane, alcoholicsolvents such as ethanol, butanol, and 1-methoxy-2-propanol, or othersolvents. Preferred solvents are the alcoholic or ethereal solvents withboiling points between 80° and 220° C. The novolak is then isolated bythe removal of solvent and the water of condensation by atmosphericdistillation followed by moderate vacuum distillation. Bothdistillations may be carried out at 200° to 230° C.

The above-discussed resins of the present invention may be mixed withphotoactive compounds to make radiation-sensitive mixtures which areuseful as positive acting photoresists. The preferred class ofphotoactive compounds (sometimes called sensitizers) is o-quinonediazidecompounds particularly esters derived from polyhydric phenols,alkyl-polyhydroxyphenones, aryl-polyhydroxyphenones, and the like whichcan contain up to six or more sites for esterification. The mostpreferred o-quinonediazide esters are derived fromo-naphthoquinone-(1,2)-diazide-4-sulfonic acid ando-naphthoquinone-(1,2) diazide-5-sulfonic acid.

Specific examples include resorcinol1,2-naphthoquinonediazide-4-sulfonic acid esters; pyrogallol1,2-naphthoquinonediazide-5-sulfonic acid esters,1,2-quinonediazidesulfonic acid esters of (poly)hydroxyphenyl alkylketones or (poly)hydroxyphenyl aryl ketones such as 2,4-dihydroxyphenylpropyl ketone 1,2-benzoquinonediazide-4-sulfonic acid esters,2,4,dihydroxyphenyl hexyl ketone 1,2-naphthoquinonediazide-4-sulfonicacid esters, 2,4-dihydroxybenzophenone1,2-naphthoquinonediazide-5-sulfonic acid esters, 2,3,4-trihydroxyphenylhexyl ketone, 1,2-naphthoquinonediazide-4-sulfonic acid esters,2,3,4-trihydroxybenzophenone 1,2-naphthoquinonediazide-4-sulfonic acidesters, 2,3,4-trihydroxybenzophenone1,2-naphthoquinonediazide-5-sulfonic acid esters, 2,4,6-trihydroxybenzophenone 1,2-naphthoquinonediazide-4-sulfonic acid esters,2,4,6-trihydroxybenzophenone 1,2-naphthoquinonediazide-5-sulfonic acidesters, 2,3,4,4'-tetrahydroxybenzophenone1,2-naphthoquinonediazide-5-sulfonic acid esters,2,3,4,4'-tetrahydroxybenzophenone 1,2-naphthoquinonediazide-4-sulfonicacid esters, 2,2',3,4',6'-pentahydroxybenzophenone1,2-naphthoquinonediazide-5 -sulfonic acid esters, and2,3,3',4,4',5'-hexahydroxybenzophenone1,2-naphthoquinonediazide-5-sulfonic acid esters;1,2-quinonediazidesulfonic acid esters ofbis[(poly)hydroxyphenyl]alkanes such as bis(p-hydroxyphenyl)-methane1,2-naphthoquinonediazide-4-sulfonic acid esters,bis(2,4-dihydroxyphenyl)methane 1,2-naphthoquinonediazide-5-sulfonicacid esters, bis(2,3,4-trihydroxyphenyl)methane1,2-naphthoquinonediazide-5-sulfonic acid esters,2,2-bis(p-hydroxyphenyl)propane 1,2-naphthoquinonediazide-4-sulfonicacid esters, 2,2-bis(2,4-dihydroxyphenyl)propane1,2-naphthoquinonediazide-5-sulfonic acid esters, and2,2-bis(2,3,4-trihydroxyphenyl)propane1,2-naphthoquinonediazide-5-sulfonic acid esters. Besides the1,2-quinonediazide compounds exemplified above, there can also be usedthe 1,2-quinonediazide compounds described in J. Kosar, "Light-SensitiveSystems", 339-352 (1965), John Wiley & Sons (New York) or in S.DeForest, "Photoresist", 50, (1975), MacGraw-Hill, Inc. (New York). Inaddition, these materials may be used in combinations of two or more.Further, mixtures of substances formed when less than all esterificationsites present on a particular polyhydric phenol,alkyl-polyhydroxyphenone, aryl-polyhydroxyphenone, and the like havecombined with o-quinonediazides may be effectively utilized in positiveacting photoresists.

Of all the 1,2-quinonediazide compounds mentioned above,1,2-naphthoquinonediazide-5-sulfonic acid di-, tri-, tetra-, penta-, andhexa-esters of polyhydroxy compounds having at least 2 hydroxyl groups,i.e. about 2 to 6 hydroxyl groups, are most preferred. These1,2-quinonediazide compounds may be used alone or in combination of twoor more.

Among these most preferred 1,2-naphthoquinone-5-diazide compounds are2,3,4-trihydroxybenzophenone 1,2-naphthoquinonediazide-5-sulfonic acidesters, and 2,3,4,4'-tetrahydroxybenzophenone1,2-naphthoquinonediazide-5-sulfonic acid esters.

The proportion of the sensitizer compound in the radiation-sensitivemixture may preferably range from about 5 to about 40%, more preferablyfrom about 10 to about 25% by weight of the nonvolatile (e.g.,nonsolvent) content of the radiation-sensitive mixture. The proportionof total binder resin of this present invention in theradiation-sensitive mixture may preferably range from about 60% to about95%, more preferably, from about 75 to 90% of the nonvolatile (e.g.,excluding solvents) content of the radiation-sensitive mixture.

These radiation-sensitive mixtures may also contain conventionalphotoresist composition ingredients such as other resins, solvents,actinic, and contrast dyes, anti-striation agents, plasticizers, speedenhancers, and the like. These additional ingredients may be added tothe binder resin and sensitizer solution before the solution is coatedonto the substrate.

Other binder resins may also be added beside the resins of the presentinvention mentioned above. Examples include phenolic-formaldehyderesins, cresol-formaldehyde resins, phenol-cresol-formaldehyde resinsand polyvinylphenol resins commonly used in the photoresist art. Ifother binder resins are present, they will replace a portion of thebinder resins of the present invention. Thus, the total amount of thebinder resin in the radiation-sensitive composition will be from about60% to about 95% by weight of the total nonvolatile solids content ofthe radiation-sensitive composition.

The resins and sensitizers may be dissolved in a solvent or solvents tofacilitate their application to the substrate. Examples of suitablesolvents include methoxyacetoxy propane, ethyl cellosolve acetate,n-butyl acetate, diglyme, ethyl lactate, ethyl 3-ethoxy propionate,propylene glycol alkyl ether acetates, or mixtures thereof and the like.The preferred amount of solvent may be from about 50% to about 500%, orhigher, by weight, more preferably, from about 100% to about 400% byweight, based on combined resin and sensitizer weight.

Actinic dyes help provide increased resolution on highly reflectivesurfaces by inhibiting back scattering of light off the substrate. Thisback scattering causes the undesirable effect of optical notching,especially on a substrate topography. Examples of actinic dyes includethose that absorb light energy at approximately 400-460 nm [e.g., FatBrown B (C.I. No. 12010); Fat Brown RR (C.I. No. 11285);2-hydroxy-1,4-naphthoquinone (C.I. No. 75480) and Quinoline Yellow A(C.I. No. 47000)] and those that absorb light energy at approximately300-340 nm [e.g., 2,5-diphenyloxazole (PPO-Chem. Abs. Reg. No. 92-71-7)and 2-(4-biphenyl)-6-phenylbenzoxazole (PBBO-Chem. Abs. Reg. No.17064-47-0)]. The amount of actinic dyes may be up to 10% weight levels,based on the combined weight of resin and sensitizer.

Contrast dyes enhance the visibility of the developed images andfacilitate pattern alignment during manufacturing. Examples of contrastdye additives that may be used together with the radiation-sensitivemixtures of the present invention include Solvent Red 24 (C.I. No.26105), Basic Fuchsin (C.I. 42514), Oil Blue N (C.I. No. 61555), andCalco Red A (C.I. No. 26125) up to 10% weight levels, based on thecombined weight of resin and sensitizer.

Anti-striation agents level out the photoresist coating or film to auniform thickness. Anti-striation agents may be used up to 5% weightlevels, based on the combined weight of resin and sensitizer. Onesuitable class of anti-striation agents is nonionic silicon-modifiedpolymers. Nonionic surfactants may also be used for this purpose,including, for example, nonylphenoxy poly(ethyleneoxy) ethanol;octylphenoxy (ethyleneoxy) ethanol; and dinonyl phenoxypoly(ethyleneoxy) ethanol.

Plasticizers improve the coating and adhesion properties of thephotoresist composition and better allow for the application of a thincoating or film of photoresist which is smooth and of uniform thicknessonto the substrate. Plasticizers which may be used include, for example,phosphoric acid tri-(B-chloroethyl)-ester; stearic acid; dicamphor;polypropylene; acetal resins; phenoxy resins; and alkyl resins up to 10%weight levels, based on the combined weight of resin and sensitizer.

Speed enhancers tend to increase the solubility of the photoresistcoating in both the exposed and unexposed areas, and thus, they are usedin applications where speed of development is the overridingconsideration even though some degree of contrast may be sacrificed,i.e., in positive resists while the exposed areas of the photoresistcoating will be dissolved more quickly by the developer, the speedenhancers will also cause a larger loss of photoresist coating from theunexposed areas. Speed enhancers that may be used include, for example,picric acid, nicotinic acid, or nitrocinnamic acid at weight levels ofup to 20%, based on the combined weight of resin and sensitizer.

The prepared radiation-sensitive resist mixture, can be applied to asubstrate by any conventional method used in the photoresist art,including dipping, spraying, whirling, and spin coating. When spincoating, for example, the resist mixture can be adjusted as to thepercentage of solids content in order to provide a coating of thedesired thickness given the type of spinning equipment and spin speedutilized and the amount of time allowed for the spinning process.Suitable substrates include silicon, aluminum, or polymeric resins,silicon dioxide, doped silicon dioxide, silicon resins, galliumarsenide, silicon nitride, tantalum, copper, polysilicon, ceramics, andaluminum/copper mixtures.

The photoresist coatings produced by the above described procedure areparticularly suitable for application to thermally grown silicon/silicondioxide-coated wafers such as are utilized in the production ofmicroprocessors and other miniaturized integrated circuit components. Analuminum/aluminum oxide wafer can be used as well. The substrate mayalso comprise various polymeric resins especially transparent polymerssuch as polyesters and polyolefins.

After the resist solution is coated onto the substrate, the coatedsubstrate is baked at approximately 70° to 125° C. until substantiallyall the solvent has evaporated and only a uniform radiation-sensitivecoating remains on the substrate.

The coated substrate can then be exposed to radiation, especiallyultraviolet radiation, in any desired exposure pattern, produced by useof suitable masks, negatives, stencils, templates, and the like.Conventional imaging process or apparatus currently used in processingphotoresist-coated substrates may be employed with the presentinvention. In some instances, a post-exposure bake at a temperatureabout 10° C. higher than the soft bake temperature is used to enhanceimage quality and resolution.

The exposed resist-coated substrates are next developed in an aqueousalkaline developing solution. This solution is preferably agitated, forexample, by nitrogen gas agitation. Examples of aqueous alkalinedevelopers include aqueous solutions of tetramethylammonium hydroxide,sodium hydroxide, potassium hydroxide, ethanolamine, choline, sodiumphosphates, sodium carbonate, sodium metasilicate, and the like. Thepreferred developers for this invention are aqueous solutions of eitheralkali metal hydroxides, phosphates or silicates, or mixtures thereof,or tetramethylammonium hydroxide.

Alternative development techniques such as spray development or puddledevelopment, or combinations thereof, may also be used.

The substrates are allowed to remain in the developer until all of theresist coating has dissolved from the exposed areas. Normally,development times from about 10 seconds to about 3 minutes are employed.

After selective dissolution of the coated wafers in the developingsolution, they are preferably subjected to a deionized water rinse tofully remove the developer or any remaining undesired portions of thecoating and to stop further development. This rinsing operation (whichis part of the development process) may be followed by blow drying withfiltered air to remove excess water. A post-development heat treatmentor bake may then be employed to increase the coating's adhesion andchemical resistance to etching solutions and other substances. Thepost-development heat treatment can comprise the baking of the coatingand substrate below the coating's thermal deformation temperature.

In industrial applications, particularly in the manufacture ofmicrocircuitry units on silicon/silicon dioxide-type substrates, thedeveloped substrates may then be treated with a buffered, hydrofluoricacid etching solution or plasma gas etch. The resist compositions of thepresent invention are believed to be resistant to a wide variety of acidetching solutions or plasma gases and provide effective protection forthe resist-coated areas of the substrate.

Later, the remaining areas of the photoresist coating may be removedfrom the etched substrate surface by conventional photoresist strippingoperations.

The present invention is further described in detail by means of thefollowing Examples. All parts and percentages are by weight unlessexplicitly stated otherwise.

EXAMPLE 1

A novolak resin was prepared in a round bottom flask by the acidiccondensation of 2,2"-bis(4-hydroxy-3-methylphenyl)propane (BHMPP) witho-cresol bismethylol (BHMOC). Thus, BHMPP (10.00 g, 39.0 mmoles) andBHMOC (6.56 g, 39.0 mmoles) were added to a round bottom flask (125 mL).To this mixture were added 1-methoxy-2-propanol (5.00 g) as solvent andoxalic acid (0.17 g, 1 weight percent relative to phenolics) as theacidic reaction catalyst.

The reaction flask was then heated in a 140° C. oil bath and stirred. Asthe mixture heated, it became a homogeneous, clear, light yellowsolution. The polymerization reaction proceeded for 20 hours. After thattime the oil temperature was increased to 230° C. to atmosphericallydistill off the water of condensation as well as the solvent, and todecompose the catalyst. Moderate vacuum was then applied for a shorttime to ensure removal of volatiles.

After cooling, 14.06 g of light yellow, glassy polymer were isolated.This corresponds to a 93% yield based on theoretical weight of polymerexpected (15.16 g) for 100% condensation of the monomers.

The time to clear (T_(c)) and molecular weight data for this polymer aregiven in Table 1. The theoretical dimer and trimer block content of eachnovolak were calculated. These calculations are given in Table 2 below.

EXAMPLE 2

The procedure of Example 1 was repeated except that BHMPP (40.00 g,156.0 mmoles) was added to a round bottom flask (500 mL). To this wereadded a mixture of BHMOC (22.30 g, 132.6 mmoles) and BHMPC (3.94 g, 23.4mmoles), 1-methoxy-2-propanol (20.00 g) as solvent and oxalic acid (0.68g, 1 wt % relative to phenolics) as the acidic reaction catalyst.

The resulting light yellow glassy polymer weighed 61.60 g, whichcorrespond to 102% yield, based on theoretical weight of polymerexpected (60.64 g) for 100% condensation of the monomers.

The measured T_(c) and molecular weight data are given below in Table 1.The calculated content of dimer and trimer blocks are given in Table 2below.

EXAMPLE 3

The procedure of Example 1 was repeated except that BHMPP (40.00 g,156.0 mmoles) was added to a round bottom flask (500 mL). To this wasadded a mixture of BHMOC (20.99 g, 124.8 mmoles) and BHMPC (5.25 g, 31.2mmoles), 1-methoxy-2-propanol (20.00 g) as solvent and oxalic acid (0.68g, 1 wt % relative to phenolics) as the acidic reaction catalyst.

The resulting light yellow glassy polymer weighed 62.64 g, whichcorresponds to 103% yield, based on theoretical weight of polymerexpected (60.64 grams) for 100% condensation of the monomers.

The measured T_(c) and molecular weight data are given below in Table 1.The calculated content of dimer and trimer blocks are given in Table 2below.

EXAMPLE 4

The procedure of Example 1 was repeated except that a mixture of BHMOC(4.92 g, 29.2 mmoles) and BHMPC (1.64 g, 9.8 mmoles) were employedinstead of the pure BHMOC reactant.

The resulting light yellow glassy polymer weighed 15.2 g, whichcorresponds to 100% yield, based on theoretical weight of polymerexpected (15.16 g) for 100% condensation of the monomers.

The measured T_(c) and molecular weight data are given below in Table 1.The calculated content of dimer and trimer blocks are given in Table 2below.

EXAMPLE 5

The procedure of Example 1 was repeated except that a mixture of BHMOC(3.28 g, 19.5 mmoles) and BHMPC (3.28 g, 19.5 moles) were employedinstead of the pure BHMOC reactant.

The resulting light yellow glassy polymer weighed 11.87 g whichcorresponds to 78% yield, based on theoretical weight of polymerexpected (15.16 g) for 100% condensation of the monomers.

The measured T_(c) and molecular weight data are given below in Table 1.The calculated content of dimer and trimer blocks are given in Table 2below.

EXAMPLE 6

The procedure of Example 1 was repeated again except that pure BHMPC(6.56 g, 39.0 mmoles) were employed instead of the pure BHMOC reactantand no solvent was added.

The resulting light yellow glassy polymer weighed 11.87 g, whichcorresponds to 101% yield, based on theoretical weight of polymerexpected (15.34 g) for 100% condensation of the monomers.

The measured T_(c) and molecular weight data are given below in Table 1.The calculated content of dimer and trimer blocks are give in Table 2below.

                  TABLE 1                                                         ______________________________________                                        Methylol                                                                      Feed Stock                                                                    Example                                                                              %        %                                                             No.    BHMPC    BHMOC    T.sub.c M.sub.w                                                                            M.sub.n                                                                            M.sub.w /M.sub.n                   ______________________________________                                        1       0       100       4 sec  2707 1117 2.42                               2      15       85       119 sec 5690 1854 3.10                               3      20       80       146 sec 6278 1956 3.21                               4      25       75       150 sec 7584 2117 3.58                               5      50       50       >750 sec                                                                              9143 2502 3.65                               6      100       0       >750 sec                                                                              3512 1360 2.58                               ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Ex-                      Weight %  Weight %                                   ample Type of Methylene Bond                                                                           Trimer    Dimer                                      No.   % o--o   % o--p   % p--p Block.sup.a                                                                           Block.sup.a                            ______________________________________                                        1     33.33    33.33    33.33   0.00   66.67                                  2     38.33    28.33    33.33  15.00   56.10                                  3     40.00    26.67    33.33  20.00   52.80                                  4     41.67    25.00    33.33  25.00   50.00                                  5     50.00    16.67    33.33  50.00   33.00                                  6     66.67     0.00    33.33  100.00   0.00                                  ______________________________________                                         .sup.a Calculations are based on the assumption of infinite molecular         weight. The actual values should be slightly lower due to the degree of       polymerization less than infinite. However, they should represent             reasonable estimates.                                                    

Times to Clear

The time to clear (T_(c)) for this and the other polymers was measuredusing a dual channel development rate monitor (DRM). One micron thickfilms of the polymer were spin cast in Si/SiO₂ wafers. The coatings weredeveloped by immersion in 0.236N aqueous tetramethylammonium hydroxidesolution and the development rate was monitored by DRM. The T_(c) isdefined as the time in seconds to develop the one micron coating.

Molecular Weights

The molecular weight of the polymers was measured by gel permeationchromatography (GPC) on a Phenomenex Phenogel 10 four column set (50,100, 500, and 10,000 A). The elution solvent was tetrahydrofuran and theflow rate was 1.0 mL/min at 35° C. The molecular weights were determinedrelative to narrow polystyrene standards. The weight average (M_(w)) andnumber average (M_(n)) molecular weights as well as polymer dispersity(M_(w) /M_(n)) for these polymers is given in Table I. The GPC tracesshow significant differences relative to conventionally preparednovolak. The polymer dispersity is low (2.4 to 3.7). There arerelatively little low molecular weight species in the polymer.

Preparation of Photoresist Formulations

Photoresist formulations may be prepared by dissolving in ethyl lactatethree parts by weight of some of the alkali-soluble resins made abovewith one part photoactive compound prepared by condensation of 1 mole2,3,4,4'-tetrahydroxy-benzophenone with 2.75 moleso-napthoquinone-(1,2)-diazide-5-sulfonic acid chloride.

After mixing, the formulation may be filtered through an 0.2 micron poresize filter.

Photoresist Processing

A. Photoresist Coatings

Photoresist solutions so prepared as above may be spin-coated onto fourinch silicon wafers, which is primed with hexamethyldisilazane (HMDS).The coated wafers may be soft baked on a hot plate for 50 seconds at110° C. Uniform coatings, of about 1.2 micron in thickness may beobtained by spinning at velocities ranging from 4,000 to 6,000 RPM for30 seconds, depending upon the solution viscosity. If necessary, thesolids content may be adjusted to fit this spin speed range.

B. Exposure of Photoresist Coatings

Photoresist coatings may be exposed on a Canon G line step and repeatexposure tool equipped with a 0.43 numerical aperture lens. Thisexposure tool provides a narrow spectral output at 436 nm.

C. Development of Exposed Photoresist Coatings

The exposed photoresist coatings may be puddle developed using a 2.38%weight percent tetramethyl ammonium hydroxide aqueous developer solutionin a two second spray and 58 second dwell cycle followed by rinsing andspin drying.

D. Photoresist Performance Evaluations

The photoresist formulations may be evaluated for photospeed; line andspace resolution; scum; and profile.

The photoresists made from the novolaks of the above should exhibit goodprofiles, useful photospeeds, submicron line, and space resolution withno scum.

While the invention has been described above with reference to specificembodiments thereof, it is apparent that many changes, modifications,and variations can be made without departing from the inventive conceptdisclosed herein. Accordingly, it is intended to embrace all suchchanges, modifications, and variations that fall within the spirit andbroad scope of the appended claims. All patent applications, patents,and other publications cited herein are incorporated by reference intheir entirety.

What is claimed is:
 1. A novolak resin composition consisting of thereaction product of a para-, para-bonded bisphenol having formula (A):##STR7## wherein R₁ =lower alkyl group having 1-4 carbon atoms, halogen,or lower alkoxy group having 1-4 carbon atoms;wherein R₂ =hydrogen orlower alkyl group having 1-4 carbon atoms; and wherein X is selectedfrom the group consisting of CH₂, CH(CH₃), C(CH₃)₂, O, and S;with abismethylol monomer selected from a difunctional ortho-, ortho-phenolicbismethylol of formula (B), a difunctional ortho-, para-phenolicbismethylol of formula (C), or mixtures thereof: ##STR8## wherein R₃ isselected from CH₃, CH₂ CH₃, Cl and Br; and wherein R₄ is selected from Hand CH₃.
 2. The novolak resin of claim 1 wherein the mole ratio of themonomers of formula (A) to the combined monomers of formulae (B) and (C)is from about 1:0.8 to about 1:1.2.
 3. The novolak resin of claim 1wherein R₁ =CH₃, Cl, Br, or OCH₃ ; R₂ =H or CH₃ ; and X=CH₂, CH(CH₃), orC(CH₃)₂ in formula (A).
 4. The novolak resin of claim 1 wherein R₃ =CH₃or C(CH₂)₃ ; and R₄ =H in both formulae (B) and (C).
 5. The novolakresin of claim 1 wherein said para-, para-bonded bisphenol is2,2"-bis(4-hydroxy-3-methylphenyl) propane; wherein said difunctionalortho-, ortho phenolic bismethylol is p-cresol bismethylol; and whereinsaid para, ortho-phenolic bismethylol is o-cresol bismethylol.